This invention relates to radar apparatus.
Conventional modern radar displays generally utilize a cathode-ray tube scanned on a rectangular raster to present an image of the radar return signals, range and bearing markers and graphic information to the user. Where high resolution is required, a 1000 line raster scan is employed. Adjacent pixels along a line of a rectangular raster have the same separation over the entire screen so that the cartesian resolution over the screen is identical, but the bearing resolution is progressively lower towards the radar origin (that is, the radar-carrying ship). In many circumstances, such as when navigating a vessel in open water, this is satisfactory at some distance from the radar origin. There are, however, circumstances in which a greater bearing resolution is required at close ranges, such as when navigating in rivers, canals and estuaries and when berthing. A conventional rectangular raster-scanned radar cannot provide the degree of bearing resolution that is required.
It has been previously proposed to use a spiral-scanned display which has the advantage that the separation between adjacent pixels close to the center of the display, that is, within close range of the radar-carrying vessel, is less than at the edge of the display, because they each subtend the same angle. Such a display should provide a greater resolution in the region where it is of most importance. In practice, however, such spiral-scanned displays have not provided the desired resolution, because very high processor speeds are necessary to handle the data. For example, using a line rotation frequency of 36 KHz with 2048 bits of screen data in one rotation would require a memory read frequency of 73 MHz. To handle data at this frequency would be very difficult and prohibitively expensive in commercial applications.